Gas-Generating Additives Having Improved Shelf Lives for Use in Cement Compositions

ABSTRACT

According to various embodiments, gas-generating additives for use in a cement composition comprise: a gas-generating material at least partially coated with a mixture comprising a fatty acid ester of sorbitan, glycerol, or pentaerythritol and having a shelf life of about 12 months or greater. The gas-generating additives may also include a C 8 -C 18  hydrocarbon. In more embodiments, cement compositions comprise: a gas-generating material at least partially coated with a mixture comprising a fatty acid ester of sorbitan, glycerol, or pentaerythritol and a C 8 -C 18  hydrocarbon for increasing a shelf life of the gas-generating material. In yet more embodiments, cement compositions comprise: a cement; a fluid for making the cement composition pumpable; a hydrogen-generating material at least partially coated with a mixture for delaying a hydrogen-generating reaction, the mixture comprising sorbitan monooleate and an isoparaffin.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional patent application of U.S. patentapplication Ser. No. 11/153,087 entitled “Gas-Generating AdditivesHaving Improved Shelf Lives for Use in Cement Compositions,” filed onJun. 15, 2005, the entire disclosure of which is incorporated herein byreference.

BACKGROUND

The present invention generally relates to cementing, and moreparticularly to additives for use in cement compositions that haveimproved shelf lives, wherein the additives are initially formed tocomprise a gas-generating material at least partially encapsulated witha coating comprising a fatty acid ester of sorbitan, glycerol, orpentaerythritol and a C₈-C₁₈ hydrocarbon.

The following paragraphs contain some discussion, which is illuminatedby the innovations disclosed in this application, and any discussion ofactual or proposed or possible approaches in this Background sectiondoes not imply that those approaches are prior art.

Natural resources such as oil and gas residing in a subterraneanformation or zone are usually recovered by drilling a wellbore down tothe subterranean formation while circulating a drilling fluid in thewellbore. After terminating the circulation of the drilling fluid, astring of pipe, e.g., casing, is run in the wellbore. The drilling fluidis then usually circulated downwardly through the interior of the pipeand upwardly through the annulus, which is located between the exteriorof the pipe and the walls of the wellbore. Next, primary cementing istypically performed whereby a cement slurry is placed in the annulus andpermitted to set into a hard mass (i.e., sheath) to thereby attach thestring of pipe to the walls of the wellbore and seal the annulus.Subsequent secondary cementing operations may also be performed. Oneexample of a secondary cementing operation is squeeze cementing wherebya cement slurry is employed to plug and seal off undesirable flowpassages in the cement sheath and/or the casing.

One problem commonly encountered during the placement of a cement slurryin a wellbore is unwanted gas migration from the subterranean formationinto and through the cement slurry. Gas migration is caused by thebehavior of the cement slurry during a transition phase in which thecement slurry changes from a true hydraulic fluid to a highly viscousmass showing some solid characteristics. When first placed in theannulus, the cement slurry acts as a true liquid and thus transmitshydrostatic pressure. However, during the transition phase, certainevents occur that cause the cement slurry to lose its ability totransmit hydrostatic pressure. One of those events is the loss of fluidfrom the slurry to the subterranean zone. Another event is thedevelopment of static gel strength, i.e., stiffness, in the slurry. As aresult, the pressure exerted on the formation by the cement slurry fallsbelow the pressure of the gas in the formation such that the gas beginsto migrate into and through the cement slurry. Eventually the gelstrength of the cement slurry increases to a value sufficient to resistthe pressure exerted by the gas in the formation against the slurry.

The flow channels formed in the cement during such gas migrationundesirably remain in the cement once it has set. Those flow channelscan permit further migration of fluid through the cement. Thus, thecement residing in the annulus may be ineffective at maintaining theisolation of the subterranean formation. As such, gas may undesirablyleak to the surface or to other subterranean formations. An expensiveremedial squeeze cementing operation may be required to prevent suchleakage. However, the gas leakage may further cause high volumeblow-outs shortly after cement replacement and before the cement hasinitially set.

In an effort to suppress gas migration, cement slurries have beendesigned that include metal particles such as an aluminum powder forgenerating a stabilized, dispersed gas. The gas is often generated insitu in a cement slurry by reacting the metal particles with an alkalinesolution, e.g., the cement slurry, and/or water to yield hydrogen. Asufficient amount of gas is formed in the cement slurry to prevent themigration of gas into or through the slurry before it has sufficientlygelled to resist such migration.

The metal particles contained in the cement slurry are usuallyencapsulated with an inhibitor for delaying the hydrogen-generatingreaction until a desired time such as after the slurry has been placedin its desired location in the wellbore, e.g., the annulus. Ideally, theinhibitor effectively inhibits the particles from interacting andreacting with oxygen, water vapor, and the cement slurry until gasgeneration is desired. Examples of chemical reaction inhibitors commonlyused to encapsulate the reactant metal particles, particularly aluminumpowder, are fatty acids of sorbitan, glycerol, and/or pentaerythritolsuch as sorbitan monooleate. Additional information relating to the useof metal particles to generate gas in cement slurries and/or inhibitorsto retard the generation of the gas may be found in U.S. Pat. Nos.5,718,292, 4,565,578, 4,450,010, 4,367,093, and 4,340,427, and in U.S.Patent Application Publication No. 2004/0221990 A1, each of which isincorporated herein by reference.

Unfortunately, metal particles coated with such inhibitors suffer fromthe drawback of undergoing severe sintering when they are not flowablesuch as when they are being stored. As used herein, “sintering” refersto the agglomeration of metal powders at temperatures below the meltingpoint. Such sintering may be facilitated by the non-uniformity of theinhibitor coating, mechanical vibration of the particles such as whenthey are being transported, the compaction of the particles in acontainer, and/or the exposure of the particles to relatively hightemperatures, air, oxygen, and/or moisture. As a result of suchsintering, the metal particles are neither free flowing as before norproperly encapsulated with the inhibitor, making the particles extremelyreactive. They may react with water vapor and release the hydrogenprematurely, or they may bond with oxygen to form metal oxides,precluding them from later forming hydrogen gas. The duration for whichthe particles can be stored without undergoing any changes in theirphysical (e.g., free flowing nature) or chemical properties, which isknown as the shelf life of the particles, thus is often shorter thandesired. A need therefore exists to develop an improved way of delayingthe reaction of the metal particles and thereby improve the shelf lifeof such particles.

SUMMARY

Some teachings and advantages found in the present application aresummarized briefly below. However, note that the present application maydisclose multiple embodiments, and not all of the statements in thissection necessarily relate to all of those embodiments. Moreover, noneof these statements limit the claims in any way.

Gas-generating additives for use in a cement composition comprise: agas-generating material at least partially encapsulated by a coatingcomprising a fatty acid ester of sorbitan, glycerol, or pentaerythritoland having a shelf life of about 12 months or greater. The coating mayalso include a C₈-C₁₈ hydrocarbon for increasing the shelf life of thegas-generating material. In an embodiment, the C₈-C₁₈ hydrocarbonprimarily comprises an aliphatic hydrocarbon. The gas-generatingadditives may be used in cement compositions to generate gas thereinafter the compositions have been placed in a wellbore. The coatingsurrounding the gas-generating material serves to delay the reaction forproducing the gas until desired. The gas may serve to inhibit gasmigration from an adjacent subterranean formation into and through thecement composition before it sets into a hard mass.

Coating the gas-generating material can ensure that it can be stored fora relatively long period of time (e.g., up to 1 year or longer) withoutbeing concerned that it might experience sintering and thus loose itsfree flowing nature and react prematurely. Without being limited bytheory, it is believed that the C₈-C₁₈ hydrocarbon acts as a thinner todilute the fatty acid ester of sorbitan, glycerol, or pentaerythritol,thus providing for a more uniform coating of the gas-generating materialwith the mixture. The C₈-C₁₈ hydrocarbon is hydrophobic in nature. Thus,in some embodiments, it may enhance the ability of the coating toprotect the gas-generating material from contacting water while it isbeing stored. However, it is understood that during the coatingprocedure, the whole mixture may reach temperatures higher than theambient temperature due to mechanical reasons. Consequently, a portionof the C₈-C₁₈ hydrocarbon may evaporate depending upon its vaporizationtemperature (usually increases with increasing molecular weight i.e.,from C₈ to C₁₈) and the temperature reached during coating, leaving therelatively uniform coating of the fatty acid ester of sorbitan,glycerol, or pentaerythritol to protect the gas-generating material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a side plan view of a drill rig and a wellbore forrecovering oil or gas from a subterranean formation penetrated by thewellbore.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Gas-generating additives for use in cement compositions include agas-generating material at least partially encapsulated with a coatingcomprising one or more fatty acid esters of sorbitan, glycerol, and/orpentaerythritol and initially one or more C₈-C₁₈ hydrocarbons forincreasing the shelf life of the gas-generating material. The term“shelf life” is known in the art as meaning the duration for which thegas-generating material can be stored without undergoing any significantchanges in either its physical (e.g., its free flowing nature) orchemical properties. In various embodiments, the shelf life may beincreased to in a range of, for example, from greater than about 6months to about 12 months. In other embodiments, the shelf life may beincreased to 12 months or greater. Thus, the coated gas-generatingmaterial may be stored without losing its free flowing nature and itsability to generate gas until it is time to prepare the cementcompositions. It is understood that during the coating procedure, thetemperature of the whole coating mixture may exceed the ambienttemperature due to mechanical reasons, e.g., grinding of thegas-generating particles. As a result, at least a portion of the C₈-C₁₈evaporate due to its temperature reaching its vaporization temperature(usually increases with increasing molecular weight i.e., from C₈ toC₁₈) or higher during the coating procedure. Thus, only the relativelyuniform coating of the fatty acid ester of sorbitan, glycerol, orpentaerythritol may remain to protect the gas-generating material.

The gas-generating additives may be included in cement compositions thatalso comprise cement and fluid. Various types of cements are known inthe art and may be used in the cement compositions. The cement may be ahydraulic cement composed of calcium, aluminum, silicon, oxygen, and/orsulfur which sets and hardens by reaction with water. Examples ofhydraulic cements include but are not limited to Portland cements,pozzolan cements, gypsum cements, high alumina content cements, silicacements, and high alkalinity cements. In some embodiments, the cementmay be a class A, B, C, G, or H Portland cement. The cement compositionsmay also include a sufficient amount of fluid to form a pumpablecementitious slurry. Examples of suitable fluids include but are notlimited to fresh water or salt water, e.g., an unsaturated aqueous saltsolution or a saturated aqueous salt solution such as brine or seawater.In some embodiments, the water may be present in the cement compositionsin an amount in the range of from about 33% to about 200% by weight ofthe cement (bwoc), alternatively from about 35% to about 60% bwoc.

The gas-generating material is desirably capable of generating gas suchas hydrogen (H2) via a chemical reaction. In various embodiments, thegas-generating material may comprise one or more metals that react withaqueous alkaline solutions or water to produce hydrogen. Examples ofsuitable metals include but are not limited to aluminum, calcium, zinc,magnesium, lithium, sodium, potassium, and combinations thereof. In someembodiments, the hydrogen-generating material is an aluminum powder.Examples of suitable commercial aluminum powders include SUPER CBLpowder and GAS CHECK powder, both of which are available fromHalliburton Energy Services, Inc. (HES). The amount of thegas-generating material included in the cement composition may beselected based on the amount of gas production required to preventformation gas from migrating from a subterranean formation into thecement composition while it is being placed in a wellbore. The amount ofgas-generating material required to yield a specified volume percent ofgas in the cement composition increases with pressure. For example,about 0.6% bwoc of an aluminum powder coated with the mixture describedabove is required to produce about 5% of hydrogen gas by volume of thecement composition in the case of an American Petroleum Institute (API)casing schedule of 6,000 feet. Further, about 1.10% bwoc of the coatedaluminum powder is required to produce the same volumetric amount ofhydrogen gas in the case of an API casing schedule of 14,000 feet. Thesecomparisons are based upon the use of a neat cement slurry having aninitial compressibility of 28 (μv/v)/atm.

The coating employed to encapsulate the gas-generating material mayserve as an inhibitor that delays the release of the gas in the cementcomposition until a desired time. Otherwise, the reaction of thegas-generating material to produce gas may occur rapidly, causing thegas to be released prior to the desired time, for example, prior toplacing the cement composition in the annulus of a wellbore. Moreover,hydrogen gas is highly explosive and thus its generation atinappropriate times may be dangerous. The coating may initially beformed to include from about 3% to about 10%, or alternatively fromabout 4% to about 5%, of the one or more fatty acid esters of sorbitan,glycerol, and/or pentaerythritol, all percentages being by weight of thegas-generating material. It may further initially include from about0.25% to about 5%, or alternatively from about 1% to about 2%, of theone or more C₈-C₁₈ hydrocarbons, all percentages being by weight of thegas-generating material. Examples of suitable fatty acid esters ofsorbitan, glycerol, and/or pentaerythritol include but are not limitedto sorbitan monooleate (SMO), sorbitan monoricinoleate, sorbitanmonotallate, sorbitan monoisostearate, sorbitan monostearate, sorbitandioleate, sorbitan trioleate, glycerol monoricinoleate, glycerolmonostearate, pentaerythritol monoricinoleate, and combinations thereof.Examples of suitable C₈-C₁₈ hydrocarbons include but are not limited toisoparaffins such as IA-35 synthetic isoparaffin and EXPAR M syntheticisoparaffin, which are commercially available from EXPO ChemicalCompany, Inc. of Houston, Tex.

The inhibitor optionally may also include an anti-oxidant to make thegas-generating material less susceptible to reaction with oxygen (O₂).Otherwise, the atoms of the gas-generating material might bond withoxygen atoms to form an oxide, limiting the ability of thegas-generating material to later react with the cement composition andproduce gas downhole. The anti-oxidant may be, for example,butylhydroxytoluene (BHT), butylated hydroxyanisole (BHA) andtert-butylhydroquinone (TBHQ). The amount of the anti-oxidant present inthe mixture for coating the gas-generating material may range from about0.01% to about 2.0% by weight of the gas-generating material, oralternatively from about 0.01% to about 1%.

As deemed appropriate by one skilled in the art, additional additivesmay be added to the cement compositions for improving or changing theproperties of the cement compositions. Examples of suitable additivesinclude but are not limited to fluid loss control agents, weightingagents, de-foamers, dispersing agents, set accelerators, and formationconditioning agents.

The gas-generating material may be prepared by first mixing together thecomponents of the inhibitor, followed by coating the gas-generatingmaterial with the resulting liquid mixture. In some embodiments, thecoating of the gas-generating material may be accomplished by mixing itwith the liquid mixture such that it is thoroughly contacted and wettedwith the mixture. In alternative embodiments, the liquid mixture may besprayed onto the surface of the gas-generating material. As a result,the gas-generating material is entirely, or at least partially, coatedwith the mixture. The gas-generating material may be ground into a finepowder during this coating procedure. The coated gas-generating materialmay then be stored either off-site or on-site near where it is to laterbe used in a cement composition. The coating desirably prevents thegas-generating material from prematurely reacting while it is beingstored and, if formed off-site, during its transport to the on-sitelocation. When the time comes to form a cement composition, the coatedgas-generating material may be dry blended with the cement, followed bymixing the resulting dry blend with water to form a pumpable cementslurry. Alternatively, the coated gas-generating material may beintroduced to the mix water before it is combined with the cement toform a cement slurry.

FIG. 1 illustrates using a cement composition comprising the coatedgas-generating material described herein. An oil rig 40 may bepositioned near the surface of the earth 42 for later recovering oilfrom a subterranean formation (not shown). A wellbore 44 may be drilledin the earth 42 such that it penetrates the subterranean formation. Apipe 52, e.g., a casing, may extend down through wellbore 44 fordelivering fluid to and/or from the wellbore. In a primary cementingprocess, the cement composition may be pumped down through pipe 52 andup through the annulus of wellbore 44 as indicated by arrows 46 usingone or more pumps 54. The cement composition may be allowed to setwithin the annulus, thereby sealing wellbore 44. Any secondary cementingoperations known in the art may also be performed using the cementcomposition. For example, a squeeze cementing technique may be employedto plug permeable areas or voids in the cement sheath or the pipe 52.

The inhibitor employed to coat the gas-generating material desirablydelays the reaction by which the gas-generating material produces gas,e.g., hydrogen, until the cement composition has been placed in itsdesired location in the wellbore and before or during a transition timeof the cement composition. The placement time of the cement slurry mayvary with well depth, hole size, casing size, and placement rate. It istypically in the range of from about 15 minutes to about 300 minutes. Inembodiments in which the gas-generating material is aluminum such as thefinely ground SUPER CBL aluminum powder, the reaction by which itproduces hydrogen relies on the alkalinity of the cement composition andgenerally proceeds according to the following reaction:

2Al_((s))+2OH^(˜) _((aq))+6H₂O→2Al(OH)₄ ^(˜) _((aq))+3H_(2(g))

The particular reaction rate delay that results from coating thegas-generating material with the inhibitor depends on various factors,including the properties of the gas-generating material, the downholeconditions, the composition of the cement composition, and so forth. Thereaction rate increases with increasing temperature and decreases withincreasing pressure. The reaction may be delayed for an initial timeperiod of from about 15 minutes to about 90 minutes during which thecoating either slowly dissolves or the reactants undergo diffusionthrough the coating. The reaction rate then slowly increases to a peakreaction rate for a period of from about 30 minutes to about 300minutes.

EXAMPLES

The invention having been generally described, the following examplesare given as particular embodiments of the invention and to demonstratethe practice and advantages thereof. It is understood that the examplesare given by way of illustration and are not intended to limit thespecification or the claims to follow in any manner.

Example 1

A test sample was prepared by coating SUPER CBL aluminum powder with 4%SMO and 2% IA-35 isoparaffin by weight of the SUPER CBL aluminum powder.Its shelf life was accelerated to allow this example to be carried outin a short period of time. That is, the test sample was placed in aplastic cell, and that cell was then placed in a vibrating water bath.Subsequently, air saturated with water vapor was passed through the cellwhile the bath was maintained at a higher temperature of 120° F.Therefore, the four ingredients, i.e., water vapor, oxygen, heat, andmechanical energy, required to accelerate the aging process wereprovided. The test sample survived 5 weeks without becoming veryreactive and still remains usable.

Example 2

A test sample was prepared by coating SUPER CBL aluminum powder with 4%SMO and 0.5% BHT by weight of the SUPER CBL aluminum powder. It was thentested in the same manner as the test sample in Example 1. This testsample also survived 5 weeks without becoming very reactive and stillremains usable.

Comparative Example 1

A conventional control sample was prepared by coating SUPER CBL aluminumpowder with 4% SMO by weight of the SUPER CBL aluminum powder. It wasthen tested in the same manner as the test sample in Example 1. Thistest sample became too reactive to remain usable after 3 weeks. Thetypical shelf life of a conventional SUPER CBL aluminum powder coatedwith 4% SMO is about 6 months when its aging process is not accelerated.

Based on the foregoing examples, using IA-35 isoparaffin or a BHTanti-oxidant in combination with the SMO forms a better coating for thealuminum powder by improving the shelf life of that powder. Therefore,an aluminum powder coated in this manner may serve as a very goodgas-generating material in a cement composition.

In various embodiments, methods of cementing in a wellbore comprise:coating a gas-generating material with a mixture comprising a fatty acidester of sorbitan, glycerol, or pentaerythritol and a C₈-C₁₈ hydrocarbonfor increasing a shelf life of the gas-generating material; preparing acement composition comprising the gas-generating material; introducingthe cement composition into a wellbore; and allowing the cementcomposition to set.

In additional embodiments, methods of cementing in a wellbore comprise:coating a gas-generating material with a mixture comprising a fatty acidester of sorbitan, glycerol, or pentaerythritol and a C₈-C₁₈hydrocarbon, thereby delaying the generation of a gas; preparing acement composition by combining a cement, a fluid for making the cementcomposition pumpable, and the gas-generating material; displacing thecement composition into the wellbore; allowing the gas-generatingmaterial to generate the gas within the cement composition; and allowingthe cement composition to set.

According to various embodiments, gas-generating additives for use in acement composition comprise: a gas-generating material at leastpartially encapsulated by a coating comprising a fatty acid ester ofsorbitan, glycerol, or pentaerythritol and having a shelf life of about12 months or greater. In more embodiments, cement compositions comprise:a gas-generating material at least partially coated with a mixturecomprising a fatty acid ester of sorbitan, glycerol, or pentaerythritoland a C₈-C₁₈ hydrocarbon for increasing a shelf life of thegas-generating material. In yet more embodiments, cement compositionscomprise: a cement; a fluid for making the cement composition pumpable;a hydrogen-generating material at least partially coated with a mixturefor delaying a hydrogen-generating reaction, the mixture comprisingsorbitan monooleate and an isoparaffin.

Modifications and Variations

The foregoing methods of cementing a wellbore may be applied to varioustypes of wells, including injection wells, single production wells suchas oil and gas wells, and multiple completion wells.

While preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit and teachings of the invention. Theembodiments described herein are exemplary only, and are not intended tobe limiting. Many variations and modifications of the inventiondisclosed herein are possible and are within the scope of the invention.

Accordingly, the scope of protection is not limited by the descriptionset out above but is only limited by the claims which follow, that scopeincluding all equivalents of the subject matter of the claims. Each andevery claim is incorporated into the specification as an embodiment ofthe present invention. Thus, the claims are a further description andare an addition to the preferred embodiments of the present invention.The discussion of a reference herein is not an admission that it isprior art to the present invention, especially any reference that mayhave a publication date after the priority date of this application. Thedisclosures of all patents, patent applications, and publications citedherein are hereby incorporated by reference, to the extent that theyprovide exemplary, procedural, or other details supplementary to thoseset forth herein.

1. A gas-generating additive for use in a cement composition,comprising: a gas-generating material at least partially encapsulated bya coating comprising a fatty acid ester of sorbitan, glycerol, orpentaerythritol for increasing a shelf life of the gas-generatingmaterial.
 2. The gas-generating additive of claim 1, wherein thegas-generating material comprises a hydrogen-generating material.
 3. Thegas-generating additive of claim 1, wherein the gas-generating materialcomprises aluminum, calcium, zinc, magnesium, lithium, sodium,potassium, or combinations thereof.
 4. The gas-generating additive ofclaim 1, wherein the gas-generating material comprises an aluminumpowder.
 5. The gas-generating additive of claim 1, wherein the shelflife is about 12 months or greater.
 6. The gas-generating additive ofclaim 1, wherein the shelf life is in a range of about 6 months to about12 months.
 7. The gas-generating additive of claim 1, wherein the fattyacid ester of sorbitan, glycerol, or pentaerythritol comprises sorbitanmonooleate, sorbitan monoricinoleate, sorbitan monotallate, sorbitanmonoisostearate, sorbitan monostearate, sorbitan dioleate, sorbitantrioleate, glycerol monoricinoleate, glycerol monostearate,pentaerythritol monoricinoleate, or combinations thereof.
 8. Thegas-generating additive of claim 1, wherein the mixture comprises thefatty acid ester in an amount of about 3% to about 10% by weight of thegas-generating material.
 9. The gas-generating additive of claim 1,wherein the mixture at least partially coated on the gas-generatingmaterial further comprises a C₈₋₁₈ hydrocarbon.
 10. The gas-generatingadditive of claim 10, wherein the C₈₋₁₈ hydrocarbon comprises anisoparaffin.
 11. The gas-generating additive of claim 10, wherein themixture comprises the C₈₋₁₈ hydrocarbon in an amount of about 0.25% toabout 5% by weight of the gas-generating material.
 12. Thegas-generating additive of claim 1, wherein the mixture furthercomprises an anti-oxidant.
 13. A gas-generating additive for use in acement composition, comprising: a gas-generating material at leastpartially encapsulated by a coating comprising a fatty acid ester ofsorbitan, glycerol, or pentaerythritol and a C₈₋₁₈ hydrocarbon in anamount of about 0.25% to about 5% by weight of the gas-generatingmaterial.
 14. The gas-generating additive of claim 13, wherein thegas-generating material comprises an aluminum powder.
 15. Thegas-generating additive of claim 13, wherein the shelf life is about 12months or greater.
 16. The gas-generating additive of claim 13, whereinthe fatty acid ester of sorbitan, glycerol, or pentaerythritol comprisessorbitan monooleate, sorbitan monoricinoleate, sorbitan monotallate,sorbitan monoisostearate, sorbitan monostearate, sorbitan dioleate,sorbitan trioleate, glycerol monoricinoleate, glycerol monostearate,pentaerythritol monoricinoleate, or combinations thereof.
 17. Thegas-generating additive of claim 13, wherein the mixture comprises thefatty acid ester in an amount of about 3% to about 10% by weight of thegas-generating material.
 18. The gas-generating additive of claim 13,wherein the C₈₋₁₈ hydrocarbon comprises an isoparaffin.
 19. Thegas-generating additive of claim 13, wherein the mixture furthercomprises an anti-oxidant.
 20. A gas-generating additive for use in acement composition, comprising: aluminum powder at least partiallyencapsulated by a coating comprising a fatty acid ester of sorbitan,glycerol, or pentaerythritol in an amount of about 3% to about 10% byweight of the aluminum power and an isoparaffin in an amount of about0.25% to about 5% by weight of the aluminum powder.